Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 4 / 65 / c / i

One Pt(111) layer has height $d_{111}=a_0/\sqrt{3}=2.269\mathring{\mathrm{A}}$. In constant-height operation the separation above the island is therefore only $5.000-2.269=2.731\mathring{\mathrm{A}}$. With the same decay constant,Thus the ideal predicted current is about $232\mathrm{nA}$. This very large increase can exceed the useful current range or risk tip-surface contact; constant-height imaging is poorly suited to unexpected monolayer steps. Constant-current feedback can follow the height more safely, subject to its response bandwidth.